|}

Decline

Wiwaxia and Halkieria did not thrive for a long period of time and were extinct before the Cambrian was over. A possible reason for this is that during the Cambrian substrate revolution

Cambrian substrate revolution

The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is the diversification of animal burrowing during the early Cambrian period....

, the microbial mat's disappearance triggered an alternating pattern in the fauna that were present for the Halwaxiids to eat.

Wiwaxia and Odontogriphus

Ever since the debate started in 1990, the most intense part has centered round Wiwaxia

Wiwaxia

Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...

. Until 2006 only one, poor-preserved fossil of Odontogriphus had been described; but in that year a description of many new, well-preserved specimens highlighted similarities between the two organisms, making the classification of Odontogriphus equally controversial.

Conway Morris (1985) originally dismissed the earliest classification of Wiwaxia

Wiwaxia

Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...

as a polychaete

Polychaete

The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...

 worm, because he thought there was little structural similarity between a polychaete's scale-like elytra and Wiwaxia’s sclerite

Sclerite

A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....

s, and because the arrangement of the sclerites, with quite different numbers in each band, showed no sign of the regular segmentation

Segmentation

Segmentation may mean:*Market segmentation, in economics and marketingBiology*A process of morphogenesis that divides a metazoan body into a series of semi-repetitive segments*Segmentation , a series of semi-repetitive segments...

 that is a feature of polychaetes. Instead he thought Wiwaxia was very similar to shell-less aplacophora

Aplacophora

Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The group comprises the two clades Solenogastres and Caudofoveata , which between them contain 28 families and about 320 species...

n molluscs, that it must have moved on a mollusc-like muscular foot, and that its feeding apparatus looked like a primitive form of the molluscan radula

Radula

The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

, a tooth-bearing chitin

Chitin

Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

ous "tongue". Hence he classified it as a "sister

Cladistics

Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

" of the molluscs. When he briefly described the first articulated specimens of Halkieria

Halkieria

Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...

in 1990, Conway Morris wrote of "the halkieriid-wiwaxiid body plan" and that the halkieriids might be close relatives of molluscs.

Shortly after this in 1990 Butterfield published his first paper on Wiwaxia. He argued that, since Wiwaxia’s sclerites appeared to be solid, they were not similar to the hollow sclerites of halkieriids. In fact he thought they were more similar to the chitinous bristles (setae) that project from the bodies of modern annelids and in some genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 form leaf-like scales that cover the back like roof tiles - in composition, in detailed structure, in how they were attached to the body via follicle-like pockets in the skin, and in overall appearance. He also contended that Wiwaxia’s feeding apparatus, instead of being mounted in the middle of its "head", was just as likely to be mounted in two parts on the sides of the "head", an arrangement that is common in polychaetes. He therefore classified Wiwaxia as a polychaete.

Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated Wiwaxia as an ancestor or "aunt" of the polychaetes. However they also argued that Wiwaxia was fairly closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller, more numerous and hollow. They wrote that in 1994 Butterfield had found Wiwaxia sclerites that were clearly hollow. They noted that one specimen of Wiwaxia showed traces of a small shell, possibly a vestige left over from an earlier stage in the animal's evolution, and noted that one group of modern polychaetes also has what may be a vestigial shell. However they maintained that Wiwaxia’s feeding aparatus was much more like a molluscan radula. The cladogram they presented showed the halkieriids split into three groups: one as "aunts" of brachiopod

Brachiopod

Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophore

Lophophore

The lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...

; the second group of halkieriids as "aunts" of both Wiwaxia and annelid

Annelid

The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

 worms; and the earliest halkieriids as "great aunts" of all of these.

Marine biologist Amélie H. Scheltema et al. (2003) argued that Wiwaxia’s feeding apparatus was very similar to the radula

Radula

The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

s of some modern shell-less aplacophora

Aplacophora

Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The group comprises the two clades Solenogastres and Caudofoveata , which between them contain 28 families and about 320 species...

n molluscs, and that the sclerites of the two groups are very similar. They concluded that Wiwaxia was a member of a clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 that includes molluscs.

Danish zoologist Danny Eibye-Jacobsen (2004) regarded bristles as a feature shared by molluscs, annelids and brachiopods. Hence even if Wiwaxia’s sclerites closely resembled bristles, which he doubted, this would not prove that Wiwaxia’s closest relative were annelids. He also pointed out that the very different numbers of sclerites in the various zones of Wiwaxia’s body do not correspond to any reasonable pattern of segmentation. Since in his opinion Wiwaxia lacked other clearly polychaete features, he regarded this as an argument against classifying Wiwaxia as a polychaete. In his opinion there were no strong grounds for classifying Wiwaxia as a proto-annelid or a proto-mollusc, although he thought the ojections against classification as a proto-annelid were the stronger.
Caron, Scheltema et al. (2006) interpreted Odontogriphus’s feeding apparatus as a forerunner of the molluscan radula

Radula

The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

, on the grounds that: the occasional and less distinct third tooth-row looked like evidence that the animals grew replacement tooth-rows at the rear of their mouths and shed worn-out ones from the front, as happens with molluscan radulae; the isolated pairs of tooth-rows they found, not associated with body fossils but in the same relative positions as in the more complete fossils, suggested they were mounted on a fairly tough surface, like the chitin

Chitin

Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

ous "belt" of a modern radula; they even found signs that discarded tooth-rows were sometimes eaten by the animals. They thought the teeth on the feeding apparatus of both Wiwaxia and Odontogriphus strongly resembled those of a modern group of molluscs, Neomeniomorpha

Neomeniomorpha

The Solenogastres , common name solenogasters, are a subclass of worm-like, small, shell-less molluscs , the other subclass being the Caudofoveata .-Morphology:...

. On the other hand they thought Wiwaxia bore little resemblance to polychaetes as it showed no signs of segmentation, appendage

Appendage

In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...

s in front of the mouth, or "legs" – all of which are typical polychaete features. Hence they classified both Odontogriphus, and Wiwaxia

Wiwaxia

Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...

with its "nearly identical" feeding apparatus, as primitive relatives of molluscs.

Butterfield (2006) accepted that Wiwaxia and Odontogriphus were closely related but wrote, "…it certainly does not require that they be shoehorned into the same lineage. …the seemingly trivial distinction between these two taxa is exactly what is expected at the divergence points leading from a last common ancestor to extant phyla." In his opinion the feeding apparatus of these organisms, with consisted of two or at most four rows of teeth, could not perform the functions of the "belt-like" molluscan radula with its numerous tooth-rows; the different tooth-rows in both Wiwaxia and Odontogriphus tooth-rows also have noticeably different shapes, while those of molluscan radulae are produced one after the other by the same group of "factory" cells and therefore are almost identical. He also regarded lines running across the middle region of Odontogriphus fossils as evidence of external segmentation, since the lines are evenly spaced and run exactly at right angle

Right angle

In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...

s to the long axis of the body. As in his earlier papers, Butterfield emphasized the similarities of internal structure between Wiwaxia’s sclerites and the bristles of polychaetes, and the fact that polychates are the only modern organisms in which some of the bristles form a covering over the back. He therefore concluded that that both Wiwaxia and Odontogriphus were stem-group polychaetes rather than stem-group molluscs.

In 2007 Conway Morris and Caron described a new fossil, Orthrozanclus

Orthrozanclus

Orthrozanclus reburrus is a sea creature known from the Middle Cambrian Burgess shale, about one centimeter long, with long spikes protruding from its armored body...

, which had a mineralized shell like that of halkieriids, and unmineralized sclerites and long spines like those of Wiwaxia – and, like both of these, a soft underside which they interpreted as a muscular foot, and a similar arrangement of the sclerites into three concentric

Concentric

Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another...

 bands. Some of Orthrozanclus’s sclerites appear to have been hollow, as the specimen includes what look like internal casting

Casting

In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...

s. They took these features as evidence that the halkieriids, Wiwaxia and Orthrozanclus

Orthrozanclus

Orthrozanclus reburrus is a sea creature known from the Middle Cambrian Burgess shale, about one centimeter long, with long spikes protruding from its armored body...

were very closely related and formed the group "halwaxiids". However the simplest "family tree" faces an obstacle: the siphogonuchitids appear in earlier rocks and had mineralized sclerites. Hence Conway Morris and Caron found it necessary to consider two more complex family trees, concluding that "Hypothesis 1" fitted the available data better, but was sensitive to minor changes

Sensitivity analysis

Sensitivity analysis is the study of how the variation in the output of a statistical model can be attributed to different variations in the inputs of the model. Put another way, it is a technique for systematically changing variables in a model to determine the effects of such changes.In any...

 in the characteristics used:

1. Conway Morris and Caron considered this hypothesis the more likely, although it is not robust:
   • Kimberella and Odontogriphus were early, primitive molluscs, without sclerites or any kind of mineralized armor.
   • Wiwaxia, the siphogonotuchids, Orthrozanclus and Halkeria form a side-branch of the mollusc family tree, and diverged from it in that order. This would mean that: Wiwaxia was the first of them to have sclerites, which were unmineralized; the siphogonotuchids were the first to have mineralized sclerites, although their scleritomes were simpler; halkieriids then developed more complex scleritomes, and a shell at each end; in Orthrozanclus the scleritome became unmineralized again and the rear shell vanished or became so small that it has not been seen in fossils. This hypothesis faces the difficulty that siphogonotuchids appear in earlier rocks and have simpler scleritomes than the other three groups.
   • The annelids and brachiopods evolved from the other main branch of the family tree, which did not include the molluscs.
2. The alternative view is:
   • Kimberella and Odontogriphus are early, primitive lophotrochozoans.
   • The siphogonotuchids, Halkeria, Orthrozanclus and Wiwaxia form a group that is closer to the shared ancestor of annelids and brachiopods than it is to the molluscs. The siphogonotuchids are the first of the group to become distinctive, with two types of mineralized sclerites and a "shell" made of fused sclerites. Halkieriids had three types of sclerites and two one-piece shells. In Orthrozanclus the sclerites became unmineralized and in Wiwaxia the shells were lost.

In 2008 Butterfield described a set of micro-fossils dated to between and , found in the Mahto Formation in Alberta

Alberta

Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...

's Jasper National Park

Jasper National Park

Jasper National Park is the largest national park in the Canadian Rockies, spanning 10,878 km² . It is located in the province of Alberta, north of Banff National Park and west of the City of Edmonton. The park includes the glaciers of the Columbia Icefield, hot springs, lakes, waterfalls and...

 – this fossil bed is 5 – 10 M years older than the Burgess Shale

Burgess Shale

The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

 in which the only known specimens of Odontogriphus and Wiwaxia were found. Some groupings of these micro-fossils showed a consistent arrangement that he interpreted as an "articulated apparatus" with tens of closely spaced tooth rows, apparently mounted on an organic base, and with noticeable signs of wear in the rows at one end. The rows were not quite identical, but he noted that some modern aplacophoran molluscs show similar variations. He concluded that the "articulated apparatus" was a genuine molluscan radula, most similar to those of modern aplacophorans or gastropods. He then commented on the contrast between this apparatus and the two or rarely three widely spaced and more heterogeneous tooth-rows found in fossils of Odontogriphus and Wiwaxia, and concluded that Odontogriphus and Wiwaxia were unlikely to be molluscs. He also noted that Wiwaxia lacked some polychaete features which he would expect to be easily preserved in fossils, and therefore classified Wiwaxia as a stem-group annelid, in other words an evolutionary "aunt" of modern annelids.

Halkieriids and siphogonotuchids

Most of the debate has been about halkieriids' relationship to molluscs, annelid

Annelid

The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

s and brachiopod

Brachiopod

Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s. However a paper published in 2008 argued that halkieriids were closely related to chancelloriid

Chancelloriidae

The Chancelloriids are an extinct family of animal common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism...

s, which have been generally regarded as sponges.

Relationship to Molluscs, Annelids and Brachiopods

Halkieriid sclerite

Sclerite

A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....

s were known for a long time as elements of the small shelly fossils. The first articuluted specimen, with all their hard parts together, were collected in 1989 and were described briefly in 1990 by Conway Morris and Peel (1990). The same authors produced a more detailed analysis in 1995, which divided the halkieriids into three groups: one as "aunts" of brachiopod

Brachiopod

Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophore

Lophophore

The lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...

; the second group of halkieriids as "aunts" of both Wiwaxia

Wiwaxia

Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...

and annelid worms; and the earliest halkieriids as "great aunts" of all of these.

In 2003 Cohen, Holmer and Luter supported the halkieriid-brachiopod relationship, suggesting that brachiopods may have arisen from a halkieriid lineage that developed a shorter body and larger shells, and then folded itself and finally grew a stalk out of what used to be the back.

Vinther and Nielsen (2005) proposed instead that Halkieria was a crown group

Crown group

A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

 mollusc, in other words more similar to modern molluscs that to annelids, brachiopods or any intermediate groups. They argued that: Halkieria’s sclerites resembled those of the modern solenogaster aplacophora

Aplacophora

Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The group comprises the two clades Solenogastres and Caudofoveata , which between them contain 28 families and about 320 species...

n shell-less molluscs (see ), of some modern polyplacophoran molluscs, which have several shell plates, and of the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 polyplacophoran Echinochiton

Echinochiton

Echinochiton is an Ordovician chiton with hollow spines on its margins; these spines, which are unique among the chitons, have a strong organic component and show growth lines....

; Halkieria’s shells are more similar to the shells of conchifera

Conchifera

Conchifera is a taxonomic term. It is a subphylum of the phylum Mollusca. It comprises all of the shell-bearing classes of molluscs, including the snails, clams, tusk shells, chitons, ammonites, monoplacophorans, and so on....

n molluscs, since shells of both of these groups show no trace of the canals and pores seen in polyplacophoran shell plates; the bristles of brachiopods and annelids are similar to each other but not to Halkieria’s sclerites.

However Conway Morris (2006) criticized Vinther and Nielsen's classification of Halkieria as a crown group mollusc, on the grounds that the growth of the spicules in the aplacophorans and polyplacophorans is not similar to the method of growth deduced for the complex halkieriid sclerites; in particular, he said, the hollow spines of various molluscs are not at all like the halkieriid sclerites with their complex internal channels. Conway Morris repeated his earlier conclusion that halkieriids were close to the ancestors of both molluscs and brachiopods.

In their description of the newly discovered Orthrozanclus

Orthrozanclus

Orthrozanclus reburrus is a sea creature known from the Middle Cambrian Burgess shale, about one centimeter long, with long spikes protruding from its armored body...

(2007), which has similarities to both Wiwaxia and the halkieriids, Conway Morris and Caron also took account of the siphogonuchitids, a group known only from isolated mineralized sclerites that resemble those of the halkieriids. They proposed the two "family trees" described above:

1. The halkieriids formed part of a "sister" group to the molluscs. This hypothesis implies that the halwaxiids were not monophyletic, since the siphogonuchitids appear between Wiwaxia and the halkieriids. Nevertheless Conway Morris and Caron found that this phylogeny fitted the available data better, although it was not robust.
2. The siphogonuchitids plus the halwaxiids formed a "sister" group to the annelids and brachopods, while the molluscs were "aunts" of all of these.

Relationship to chancelloriids

Porter (2008) revived an early 1980s idea that the sclerites of Halkieria are extremely similar to those of chancelloriid

Chancelloriidae

The Chancelloriids are an extinct family of animal common in sediments from the Early Cambrian to the early Late Cambrian. Many of these fossils consists only of spines and other fragments, and it is not certain that they belong to the same type of organism...

s. These were sessile, bag-like, radially symmetric organisms with an opening at the top.

Since their fossils show no signs of a gut or other organs, chancelloriids were originally classified as some kind of sponge. Butterfield and Nicholas (1996) argued that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites is similar to that of fibers of spongin

Spongin

Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. They are secreted by sponge cells known as spongocytes...

, a collagen

Collagen

Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

, in modern keratose (horny) demosponge

Demosponge

The Demospongiae are the largest class in the phylum Porifera. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges...

s. However Janussen, Steiner and Zhu (2002) denied that chancellorids were sponges on the grounds that: chancellorids' hollow sclerites, probably made of aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

, were quite different from the solid, silica-based spines of demosponges; sponges have simple skins that are only one cell thick, while chancellorids' thicker, more complex skins resemble those of more complex animals. In their opinion chancellorids were at least members of the Epitheliazoa, a grade more complex than the Porifera, to which sponges belong.

Porter (2008) found that the sclerites of halkieriids and chancelloriids resemble each other at all levels: both have an internal "pulp cavity" and a thin external organic layer; the walls are made of the same material, aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

; the arrangement of the aragonite fibers is in each is the same, running mainly from base to tip but with each being closer to the surface at the end nearest the tip. It is extremely improbable that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related. This dilemma may be resolved in various ways:

• One possibility is that chancelloriids evolved from bilaterian ancestors but then adopted a sessile lifestyle and rapidly lost all unnecessary features. However the gut and other internal organs have not been lost in other bilaterians that lost their external bilateral symmetry.
• On the other hand perhaps chancelloriids are similar to the organisms from which bilaterians evolved, and the earliest bilaterians had similar sclerites. However there are no fossils of such sclerites before , while Kimberella
  Kimberella
  Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
  
  from was almost certainly a bilaterian, but shows no evidence of sclerites.
• One solution to this dilemma
  Dilemma
  A dilemma |proposition]]") is a problem offering two possibilities, neither of which is practically acceptable. One in this position has been traditionally described as "being on the horns of a dilemma", neither horn being comfortable...
  
   may be that preservation of small shelly fossils by coatings of phosphate
  Phosphate
  A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
  
  was common only for a relatively short time, during the Early Cambrian, and that coelosclerite-bearing organisms were alive several million years before and after the time of phosphatic preservation.
• Alternatively, perhaps the common ancestor of both chancelloriids and halkieriids had very similar but unmineralized coelosclerites, and some intermediate groups independently incorporated aragonite into these very similar structures.